Sensor element array for reading and processing image information

ABSTRACT

The present invention relates to a device for the input and processing of information comprising sensor elements arranged in matrix form, which are integrally formed on a substrate. Each sensor element comprises output electronics ( 3 ) and at least one photovoltaic cell (PD), which is adapted to convert an optical signal incident upon the photovoltaic cell to an electrical signal. The device according to the invention is characterized in that each sensor element is adapted to perform compensation of an offset error in the output electronics ( 3 ) forming part of the sensor element and that each sensor element is adapted to perform the said offset compensation independently of the other sensor elements in the device.

TECHNICAL SCOPE

The present invention relates to a device for the input and processingimage information comprising sensor elements arranged in matrix formwhich are integrally formed on a substrate. Each sensor elementcomprises output electronics and a photovoltaic cell, which is adaptedto convert an optical signal, incident upon the photovoltaic cell, intoan electrical signal.

PRIOR ART

Two-dimensional image sensors, which take the form of integratedcircuits, are used in image processing. Each two-dimensional imagesensor comprises a number of sensor elements, for example 256×256 sensorelements, which must take up little surface area in order to facilitatedesign as an integrated circuit. Each sensor element comprises adetector part which converts an optical signal to an electrical signalsuitable for further processing. In order to obtain a compact imagesensor, it is important that the detector part take up as little spaceas possible, whilst at the same time it must naturally produce areliable result.

A device for compensation of the image elements in a CCD camera, inwhich a CCD sensor delivers unamplified and uncompensated image elementvalues to a common serial output, where amplification and compensationare performed centrally, is already known from U.S. Pat. No. 4,875,098.In certain applications, however, there may be a need for compensationand parallel output of the image in all image points, which problem isnot solved by the said device.

A device which combines photovoltaic cell with output electronics ineach sensor element is already known from the conference paper “StandardCMOS Active Pixel Image Sensors for Multimedia Applications” by A.Dickinson, B. Ackland, El-Sayed Eid, D. Inglis, and E. Fossum, which waspresented at the “16th Conference in Advanced Research in VLSI” atChapel Hill, N.C., on March 27-29. A disadvantage to this known deviceis that variations in the constituent electrical components give rise toso-called offset errors, which manifest themselves as a fixed pattern inthe output image. The said offset errors are counteracted in columnarcompensation electronics, which means that the image must be moved fromthe sensor element before offset compensation can be performed. Incertain applications, for example where there is processor capacity inthe sensor element, this transfer constitutes a disadvantage.

DESCRIPTION OF THE INVENTION

The object of the present invention is to provide a device for the inputand processing of image information, which reduces the occurrence ofdisturbances due to offset in an output image.

This object is achieved by a device according to the invention, whichcomprises sensor elements arranged in matrix form. Each sensor elementin the device according to the invention is adapted so that compensationof an offset error in the output electronics forming part of the sensorelement is performed in the sensor element. The said offset compensationis performed in each sensor element independently of other sensorelements in the device, in case different control signals are used forthe various sensor elements.

With the design of sensor element according to the invention the offsetcompensation can be performed locally in the sensor elements using arelatively small number of components.

DESCRIPTION OF FIGURES

FIG. 1 shows a first embodiment of a sensor element according to theinvention.

FIG. 2 shows a second embodiment of a sensor element according to theinvention.

FIG. 3 shows a third embodiment of a sensor element according to theinvention.

FIG. 4 shows a fourth embodiment of a sensor element according to theinvention.

FIG. 5 shows a fifth embodiment of a sensor element according to theinvention.

FIG. 6 shows a sixth embodiment of a sensor element according to theinvention.

PREFERRED EMBODIMENTS

The invention relates to offset compensation in a sensor element whichforms part of a set of sensor elements arranged in matrix form. All thesensor elements are integrally formed on a substrate and each comprise aphotovoltaic cell and output electronics, for example an amplifier orcomparator. Because of offset in the output electronics in each sensorelement and variations between the offset error in different outputelectronics, each sensor element is adapted for local offsetcompensation in the sensor element. Offset compensation in one sensorelement can be performed independently of offset compensation in othersensor elements in the arrangement, in case different control signalsare used for each sensor element. The compensation is based oncorrelated double sampling. Sampling of a known compensation value isfirst performed during a first compensation stage, and then sampling ofan unknown value during an image information input stage. In theexamples of sensor elements shown in FIGS. 1 to 6 a known value issampled with the reset switch and the unknown value when the value ofthe output electronics is outputted. Both these values contain constanterrors, here known as offset. By subtracting the known and unknownvalue, the constant errors can be eliminated.

The invention will now be further explained with reference to thefigures which show a number of preferred embodiments of a sensor elementaccording to the invention.

The sensor element shown in FIG. 1 has two inputs, the first of whichconnects a compensating voltage U_(th) to a capacitor C by way of afirst change-over switch 1 and the second of which connects a referencevoltage U_(ref) to the same capacitor C by way of a second change-overswitch 2. The sensor element further comprises a photodiode PD, thecathode of which is connected to a known reference level, for exampleearth. The anode of the photodiode is connected to the outputelectronics and to the said capacitor C at a node N₁. In the case shownin FIG. 1, the output electronics consist of a comparator 3 comprisingtwo cascade-connected field effect transistors FET and a reset switch 4in the form of a field effect transistor FET.

With the possible connection to two different voltage levels shown inthe figure, a compensation of an offset error in the comparator can beachieved in the sensor element. In a first compensation stage the saidreset switch 4 is closed and the compensating voltage U_(th) connected.The charge in the node N₁ is determined by the offset of the comparator3. Once the capacitor C has been charged with the compensating voltage,the image information can be picked up by the sensor element. Beforeinput of the image information commences, the compensating voltageU_(th) is disconnected and the reference voltage U_(ref) connected. Thecharge in node N₁ is varied in proportion to the difference between thereference voltage U_(ref) and the compensating voltage U_(th), whichdifference can be precisely determined through the choice of the saidreference voltage and compensating voltage. When the photodiode isilluminated, this is discharged in proportion to the luminous intensity.The discharge time of the capacitor C depends on the current through thecapacitor and once compensation has been performed will depend solely onthe precision of the capacitor C and the photodiode PD. Since thecapacitor C constitutes a passive element, the variations betweencapacitors in different sensor elements can be minimised.

FIG. 2 shows a second embodiment of a sensor element according to theinvention. This sensor element differs from the sensor element shown inFIG. 1 in that a compensating voltage U_(th) and a reference voltageU_(ref) are directly connected to the photovoltaic cell PD by way of aswitch S₁. The said photovoltaic cell is capacitively connected by wayof a capacitor C to the output electronics, which in this case consistof a comparator 3 with a reset switch 4.

In a first-compensation stage the compensating voltage U_(th) isconnected, thereby setting a starting condition for charging in node N₂.The compensating voltage U_(th) is connected by the closing of a firstchange-over switch 1. After the node N₂ has obtained a startingcondition dependent on the compensating voltage, image information canbe picked up by the sensor element. Before input of the imageinformation commences, the change-over switch 1 is opened so that thecompensating voltage U_(th) is disconnected. The reference voltageU_(ref) is connected by closing of the change-over switch 2 and closingof the switch S₁ for a brief interval. The voltage in N₁ is changed fromthe compensating voltage U_(th) to the reference voltage U_(ref). Thesaid voltage change is transmitted by the capacitor C to N₂. When theswitch S₁ is opened, the charge in node N₁ is discharged by thephotocurrent through the photodiode. The comparator 3 switches over whenthe voltage in N₁ has returned to the compensating voltage U_(th). Thedischarge time is influenced solely by the photodiode PD.

FIG. 3 shows a third embodiment of a sensor element according to theinvention. The sensor element shown comprises a comparator 3 with areset switch 4 and a photodiode PD. In this embodiment use is made ofthe capacitive characteristics of the photodiode, which makes itpossible to eliminate the capacitor C shown in FIG. 1. In the same wayas in the embodiment shown in FIG. 1, two different voltage levels areused in order to bring about the desired elimination of offset errors inthe comparator 3. In a first compensation stage the reset switch 4 isclosed and the compensating voltage U_(th) is connected. The charge innode N₁ is determined by the offset of the comparator. When the desiredcharge is attained in node N₁, the compensating voltage U_(th) isdisconnected and the reference voltage U_(ref) connected. The resetswitch 4 is opened and it is possible to input the image information.The charge in node N₁ is corrected by ΔQ=(U_(ref)−U_(th))C_(PD) as inthe embodiment shown in FIG. 1 The discharge which is produced byillumination of the photodiode in this case therefore depends solely onthe precision of the photodiode. By using the capacitive characteristicsof the photodiode, the capacitor C (according to FIG. 1) can beeliminated, which may be of value from the point of view ofspace-saving. The method of offset compensation shown does not, however,give such complete offset compensation as in the embodiment shown inFIG. 1, owing to the fact that the capacitance in the photodiode is notindependent of the voltage over the diode.

FIG. 4 shows a fourth embodiment of a sensor element which permitscompensation of offset. The sensor element shown comprises a comparator5 with a reset switch 4 a, 4 b, a capacitor C and a photodiode PD. Thecomparator differs somewhat from the comparator 3, described earlier inconnection with FIGS. 1 to 3, but the problem with offset error existseven for this type of comparator. In a first compensation stage in thesensor element, the reset switch 4 a, 4 b is closed and the compensatingvoltage U_(th) is connected. The charge in the nodes N₁ and N₂ aredetermined by the switch-over point of the comparator 5. Thecompensating voltage U_(th) is then disconnected and the referencevoltage U_(ref) is connected. The reset switch is opened and it ispossible to input the image information. The charge in node N₂ iscorrected by (U_(ref)−U_(th))C. For the condition of the comparator tobe changed an equally large change must occur on the comparator's secondinput in node N₁. This change is controlled by the current through thephotodiode PD and, owing to the compensation shown, is not affected byany offset error in the comparator 5.

FIG. 5 shows a further embodiment of a sensor element with localcompensation of offset. In the sensor element shown a type of comparator5 is used which is similar to the embodiment shown in FIG. 4. Inaddition to the comparator 5, the sensor element comprises a resetswitch 6, a capacitor C and a photodiode PD. A first compensation stageis performed in the sensor element, as described for previousembodiments, by the connection of a compensating voltage U_(th). In aninput stage the reset switch 6 is opened and the reference voltageU_(ref) is connected for a brief interval. At that moment the charge innode N₁ is corrected by (U_(ref)-U_(th))C_(N1) where CN₁is the totalcapacitance in node N₁, which is essentially dependent on thecapacitance of the photodiode PD. The discharge time in this casedepends on the precision of the photodiode.

Finally, in the sensor element shown in FIG. 6 a photoresistor R₂ isused for converting optical information to an electrical signal. In acompensation stage a reset switch 2 is closed and a reference voltageU_(ref) is connected. The charge in node N₁ is determined by thecomparator's switch-over point, that is its offset. In an imageinformation input stage the reset switch is opened and U_(ref) isdisconnected. The charge in node N₁ is corrected as a function of thevoltage division between the resistors R₁ and R₂. The comparatorswitches over as a function of the polarity of the charge.

The embodiments described above represent some examples of sensorelements with local compensation of offset. The invention, however, isnot limited to these embodiments described, other sensor elements withlocal offset compensation also being possible within the scope of theidea of the invention.

What is claimed is:
 1. Device for the input and processing of imageinformation, said device comprising: a substrate; a plurality of sensorelements integrally arranged on said substrate in matrix form, each ofsaid sensor elements comprising output electronics adapted to output anelectronic signal and at least one photovoltaic cell adapted to convertan optical signal incident thereupon into an electrical signal, whereineach of said sensor elements is adapted to perform a compensation so asto substantially eliminate an offset error of said sensor, saidcompensation being performed in said output electronics, each of saidsensor elements is adapted to perform said compensation independentlyfrom other sensor elements in said device, and each of said sensorelements is adapted to perform correlated double sampling of one knownand one unknown value.
 2. Device for the input and processing of imageinformation, said device comprising: a substrate; a plurality of sensorelements integrally arranged on said substrate in matrix form, each ofsaid sensor elements comprising output electronics adapted to output anelectronic signal and at least one photovoltaic cell adapted to convertan optical signal incident thereupon into an electrical signal, whereineach of said sensor elements is adapted to perform a compensation so asto substantially eliminate an offset error of said sensor, saidcompensation being performed in said output electronics, each of saidsensor elements is adapted to perform said compensation independentlyfrom other sensor elements in said device, and each of said sensorelements comprises two change-over switches adapted to control aconnection of two different reference voltages to said sensor element.